Alpha Comae Berenices
A binary star system in Coma Berenices, faintly visible to the naked eye.
Diadem, also known as Alpha Comae Berenices, is a binary star system located about 58 light-years from Earth in the northern constellation Coma Berenices, or Berenice's Hair. Its combined brightness, which fluctuates between magnitudes 4.29 and 4.35, makes it just visible without a telescope. The system is made up of two main sequence stars, both slightly hotter and more luminous than the Sun. These two components are labeled Alpha Comae Berenices A and B, with the brighter one, A, officially bearing the name Diadem—a traditional moniker for the system.
- Bayer designation
- α Comae Berenices
- Combined apparent magnitude
- 4.29 to 4.35
- Distance
- 58 light-years (17.8 pc)
- Component magnitudes
- 5.05 and 5.08
- Orbital period
- 25.87 years
- Mean separation
- approximately 10 AU
- Maximum angular separation
- 0.7 arcsec
Lore & Background
The system's Bayer designation is α Comae Berenices, Latinized to Alpha Comae Berenices, and it is said to represent the crown worn by Queen Berenice. The proper name Diadem is of unknown modern origin. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems, and approved the name Diadem for the component Alpha Comae Berenices A on 1 February 2017. In Chinese, the system is part of the asterism Left Wall of Supreme Palace Enclosure, and its Chinese name is 太微左垣五 (Tài Wēi Zuǒ Yuán wǔ), representing The First Eastern General.
Reader's Guide
Although Alpha Comae Berenices bears the title 'alpha', at magnitude 4.32 it is actually fainter than Beta Comae Berenices. The binary has almost equal components of magnitudes 5.05 and 5.08 orbiting each other with a period of 25.87 years. The system is aligned nearly 'edge-on' from Earth so that the two stars appear to move back-and-forth in a straight line with a maximum separation of only 0.7 arcsec. The mean physical separation between them is approximately 10 AU, about the distance between the Sun and Saturn. Eclipses are predicted to occur between the two components, but they have not been successfully observed due to miscalculations of the time of eclipse. If eclipses do occur, the next primary eclipse is expected to be around 27 September 2040; the most recent date for a secondary eclipse, which had only a 3.3% probability of happening, was 11 January 2026. The binary star has a visual companion, CCDM J13100+1732C, of apparent magnitude 10.2, located 89 arcseconds away along a position angle of 345°. Alpha Comae Berenicis forms an isosceles triangle with globular star clusters Messier 53 and NGC 5053, with an apparent diameter of a little more than one degree, and its location is westward (preceding) of both globular star clusters.
Did You Know?
- The system's combined apparent visual magnitude varies from 4.29 to 4.35.
- The next primary eclipse is expected around 27 September 2040.
- Alpha Comae Berenices forms an isosceles triangle with globular clusters Messier 53 and NGC 5053.
More in Binary and Multiple Stars, Part 2 1-24
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